1. Field of the Invention
The present invention relates to a piezoelectric resonant component such as a piezo-resonator or a piezoelectric filter. More specifically, the present invention relates to a surface mount piezoelectric resonant component including an energy trap type piezoelectric resonant element.
2. Description of the Related Art
Conventionally, various types of surface mounting type piezoelectric resonant components incorporating an energy trap type piezoelectric resonant element have been proposed.
A conventional example of a piezoelectric resonant component of this type will be described with reference to FIGS. 12A and 12B. A piezoelectric resonant component 101 has a construction in which exterior substrates 103 and 104 are stacked from on top of and below a plate-like piezoelectric resonant element 102.
The piezoelectric resonant element 102 is an energy trap type piezoelectric resonant element. Resonant electrodes 102b and 102c are provided on both main surfaces of a rectangular plate-like piezoelectric substrate 102a. The resonant electrodes 102b and 102c are connected to lead electrodes 102d and 102e. The lead electrodes 102d and 102e are arranged so as to reach the side edges of the piezoelectric substrate 102a. 
The portion of the piezoelectric substrate that is opposed to the resonant electrodes 102b and 102c through the piezoelectric substrate 102a constitutes an energy trap type piezoelectric vibration portion.
The exterior substrates 103 and 104 have recesses 103a and 104a, and a space for allowing for free and unhindered vibration of the piezoelectric vibration portion is defined by the recesses 103a and 104a. As shown in FIG. 12B, the exterior substrates 103 and 104 are fixed to the piezoelectric resonant element 102 via adhesive layers 105 and 106.
Further, on the outer surface of the piezoelectric resonant component 101, external electrodes 107 through 109 are arranged so as to be wound around the upper surface, the side surfaces and the lower surface of the piezoelectric resonant component 101. The side surfaces of the external electrodes 107 and 109 are electrically connected to the lead electrodes 102d and 102e. The external electrode 108 is arranged so as to extract capacitance between the external electrodes 107 and 109.
That is, capacitors are defined between the external electrodes 107 and 108 and between the external electrodes 108 and 109.
In the piezoelectric resonant component 101, it is necessary to form spaces B and C for allowing for free and unhindered vibration of the energy trap type piezoelectric vibration portion. Thus, recesses 103a and 104a and are formed in the exterior substrates 103 and 104 as described above.
Incidentally, as in the other electric components, in the piezoelectric resonant component, there is a demand for a reduction in size and thickness. However, in the piezoelectric resonant component 101, it is necessary to provide the spaces B and C, so that when a reduction in thickness is achieved, the mechanical strength of the exterior substrates 103 and 104 deteriorates.
In particular, in the piezoelectric resonant component 101, due to the stress during assembly or mounting on the user side, cracks D and E are generated in the portion where the recesses 103a and 104a of the exterior substrates 103 and 104 are provided, where the mechanical strength is minimum, so that there is a fear of a fracture being generated in the exterior substrates 103 and 104. Thus, to prevent the generation of this fracture, it is necessary to increase the thickness of the exterior substrates 103 and 104 to some degree, which prevents reduction in thickness.
A construction is known in which, to form the spaces B and C, flat exterior substrates are used, and the thickness of the adhesive layer for gluing together the piezoelectric vibration element and the exterior substrates is increased to form the spaces B and C. Also in the case in which a space is formed due to the thickness of this adhesive, the thickness of the exterior substrates is reduced when a reduction in the thickness of the piezoelectric resonant component is achieved. Thus, similarly, to prevent the fracture of the exterior substrates, the thickness of the exterior substrates cannot reduced significantly, thereby making it difficult to achieve a reduction in thickness.
In order to overcome the problems described above, preferred embodiments of the present invention provide a piezoelectric resonant component in which it is possible to achieve a reduction in the thickness of the exterior case material and in which even when a thin exterior case material is used, fracture or chipping is prevented.
According to a preferred embodiment of the present invention, a piezoelectric resonant component includes an energy trap type piezoelectric resonant element which has a piezoelectric plate and a plurality of resonant electrodes partially disposed on both main surfaces of the piezoelectric plate and in which a piezoelectric vibration portion is defined by a portion in which the resonant electrodes of the main surfaces are opposed to each other, an exterior case member which is fixed to at least one surface of the piezoelectric resonant element so as to define a space for allowing for free and unhindered vibration of the vibration portion of the piezoelectric resonant element, and a plurality of external electrodes arranged on the surface on the opposite side of the surface of the exterior case member which is fixed to the piezoelectric resonant element, wherein the plurality of external electrodes are arranged such that the plurality of external electrodes do not overlap with the space through the intermediary of the exterior case member.
In another preferred embodiment of the present invention, a piezoelectric resonant component includes an energy trap type piezoelectric resonant element which has a piezoelectric plate and a plurality of resonant electrodes partially arranged on both main surfaces of the piezoelectric plate and in which a piezoelectric vibration portion is defined by a portion of the piezoelectric plate in which the resonant electrodes on the main surfaces are opposed to each other, an exterior case member which is fixed to at least one surface of the piezoelectric resonant element so as to define a space for allowing for free and unhindered vibration of the vibration portion of the piezoelectric resonant element, and a plurality of external electrodes arranged on the surface on the opposite side of the surface of the exterior case member which is fixed to the piezoelectric resonant element, wherein a recess is formed in the surface on the opposite side of the surface of the exterior case member which is fixed to the piezoelectric resonant element so as to be opposed to the space.
In another preferred embodiment of the present invention, the portion of the exterior case member that is opposed to the space is preferably curved toward the piezoelectric resonant element side.
In another modified preferred embodiment of the piezoelectric resonant component of the present invention, a recess is formed in the piezoelectric resonant element side surface of the exterior case member, and a space is defined by this recess.
In still another preferred embodiment of the piezoelectric resonant component of the present invention, the surface of the exterior case member which is fastened to the piezoelectric resonant element is a flat surface, and the adhesive layer joining the piezoelectric resonant element with the exterior case member is constructed so as to form the space.
It is also preferable that the exterior case member is a flat-plate-like exterior substrate, and the pair of exterior substrates are stacked on both sides of the piezoelectric resonant element.
In another modified preferred embodiment of the piezoelectric resonant component of the present invention, the exterior case member has a flat-plate-like exterior substrate and a cavity forming exterior case member having an opening on the exterior substrate side and joined to the exterior substrate from the opening side, wherein the piezoelectric resonant element is fixed to the exterior substrate or the cavity forming exterior case member and sealed in a cavity formed by the exterior substrate and the cavity forming exterior case member.
Other features, elements, characteristics, and advantages of the present invention will become apparent from the detailed description of preferred embodiments of the present invention below with reference to the attached drawings.